Printed circuit boards are today manufactured by a process which is almost completely automated. The circuits for a particular printed circuit board are generated using a computer aided design (CAD) machine which will not only generate a schematic of the printed circuit board but also will provide the board layout for all of the devices thereon. The printed circuit board layout information is provided to a device such as a laser plotter which exposes the artwork needed to fabricate the printed circuit board. The artwork comprises a series of transparent and opaque areas as features corresponding to the PCB devices.
However, defects may be present in the printed circuit board or the PCB artwork which would render the printed circuit board useless. These defects can have a variety of causes, including shrinkage in the artwork or failures in the fabrication process. Many known systems for defect detection in printed circuit boards simply compare a given printed circuit board against a reference, defect free printed circuit board (i.e. golden board) to detect errors created during the fabrication process. A Golden Board System does not detect errors in subsequent boards that were also in the golden board.
Some of the known optical PCB inspection systems are configured to examine inspection marks which are placed on the artwork away from the features. The dimensions and locations of the registration marks on the printed circuit board are compared against a reference to determine the extent of shrinkage or expansion. Should the variation in dimension exceed a certain value, the board is considered by these systems to be defective.
Most known systems used for automatic optical inspection (AOI) of PCBs are rule based systems. In a given scan line, these systems analyze scanned PCB features for their width, in pixels, and compare that measured width with one having a preferred width. To establish the existence of an error, a second class of AOI systems are edge based in that they look for state transitions across a scan line (e.g., white to black) which correspond to feature edges without constructing "features" per se.
Edge based transition systems employ a pixel to pixel comparison between the pixels in the CAD data base and those corresponding pixels that are scanned from the substrate. In order for these systems to work properly, there must be very high fidelity between the scanned and CAD data base pixels. That is; the location of the scanned pixel on the substrate should exactly correspond to that of the CAD pixel in the data base. Should this condition not be met, an error signal may be generated.
The causes of this distortion are numerous. A substrate itself may be distorted or the artwork employed may have been produced out of calibration. Other factors which yield imperfections are temperature or humidity variations from ideal conditions as well as distortion which could occur during the substrate etching process. Importantly, the distortion may not be continuous and extend entirely across a substrate or a panel, but may, instead, be localized in one or more areas. Moreover, the type, i.e., radial versus linear, may vary as can respective magnitudes. Many printed circuit board fabrication lines employ a panel containing 8-12 printed circuit boards. It is entirely conceivable that slight distortions could result in for example, an end substrate on the panel or/and circuit on the substrate being misregistered by one or more pixels.
It would be advantageous to have a method and apparatus for creating a database from the original raster CAD data for use in detection of distortion in printed circuit boards which would provide the capability of correcting for localized substrate distortion. The present invention is directed towards such a method and apparatus.